Hi Piotr, you should note that these old mics caps were not usually rated
for high RF currents. At that capacitance they consist of a stack of micas
with foil or maybe evaporated plating. Either way there is a problem with
the connection between the foil/plating and the lead-wire. One weak point
will fuse first (and the capacitance drops)and that loads more current into
the other junctions etc etc. Try one and see how it copes with the
currents...they may be rugged enough. I tried a number of old style valve
caps with little success. They were used in the outputs of Valve PAs but the
impedance is much higher so the RF currents there were much lower.
Best Wishes
Alan G3NYK
----- Original Message -----
From: "Piotr Mlynarski" <[email protected]>
To: <[email protected]>
Sent: Tuesday, January 11, 2011 8:26 PM
Subject: Re: LF: Capacitor warning and First 500kHz Class E breadboard
> Andy Talbot pisze:
> > A warning to anyone contemplating making power amps - especially class
> > E types.
> > Don't even think about using the small high voltage ceramic caps
> > availalble at low cost from the suppliers - and sold as "suitable for
> > SMPSUs etc" in resonant circuits
> > They are appalling, terrible, revolting things. You only have to
> > breath on them and they change their value.wildly
> >
> Hi Andy, LF Group
>
> Although this subject seems to be finished in some way i will shortly
> come back to this issue only because i want to tell you ( and others,
> possibly interested)
> that i have some number of mica caps (see attached
> photo=sorry-for-the-quality=made by my mobile phone)
> they are all rated at 250 Volts +/- 10% with following capacities:
> 47nF, 33nF, 22nF, 10nF, 8.2nF, 7.5nF, 6.8nF
> the total number of those mica caps is big enough for several PA's with
> some series combinations in order to increase the V rating so if you ,
Andy
> or anybody else needs them - no problem - they will go for free so,
> possibly , i will get a chance to hear more lf/mf stations... :)
>
> 73, Piotr, sq7mpj
> qth: Lodz /jo91rs/
>
>
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